1. Field of the Invention
The present invention relates to an exchange coupling film which has an antiferromagnetic layer and a ferromagnetic layer, wherein the direction of magnetization of the ferromagnetic layer is fixed in a predetermined direction by the effect of an exchange magnetic field which is generated at the interface between the antiferromagnetic layer and the ferromagnetic layer. More particularly, the present invention relates to an exchange coupling film which is improved to exhibit a large ratio of resistance variation and also to a magnetoresistive sensor, e.g., a spin valve thin-film device or an ARM device, incorporating such an exchange coupling film, as well as to a thin-film magnetic head which uses such a magnetoresistive sensor.
2. Description of the Related Art
A spin-valve-type thin-film device is a kind of GMR (Giant Magnetoresistive) device which makes use of a giant magnetoresistive effect, and is used for detecting recording magnetic fields from a recording medium such as a hard disk.
The spin-valve-type thin-film device, among various GMR devices, has advantageous features such as simplicity of the structure and high subtlety to vary its magnetic resistance even under a weak magnetic field.
The simplest form of the spin-valve-type thin-film device has an antiferromagnetic layer, a pinned magnetic layer, a non-magnetic intermediate layer, and a free magnetic layer. The antiferromagnetic layer and the pinned magnetic layer are formed in contact with each other, and the direction of the pinned magnetic layer is put into a single magnetic domain state and fixed by an exchange anisotropic magnetic field, which is produced at the interface between the antiferromagnetic layer and the pinned magnetic layer. The magnetization of the free magnetic layer is aligned in a direction which intersects the direction of magnetization of the pinned magnetic layer, by the effect of bias layers that are formed on both sides of the free magnetic layer.
Alloy films such as an Fe—Mn (Iron-Manganese) alloy film, Ni—Mn (Nickel-Manganese) alloy film and a Pt—Mn (Platinum-Manganese) alloy film are generally usable as the material of the antiferromagnetic layer, among which the Pt—Mn alloy film are attracting attention due to its advantages such as a high blocking temperature, superior corrosion resistance, and so forth. It has been recognized that, when a Pt—Mn alloy film is used as the material of a ferromagnetic layer, the film as deposited has a crystalline structure composed of face-centered cubic lattice in which atoms are positioned in an irregular manner.
In order that a large exchange coupling magnetic field is generated between a ferromagnetic layer and an antiferromagnetic layer after deposition, it is necessary that the crystalline structure of the antiferromagnetic layer be transformed from face-centered cubic lattice as disordered phase to a CuAu—I face-centered square lattice as an ordered phase. Such a transformation can be effected by a heat treatment.
It has been recognized also that a Pt—Mn alloy of bulk type is easy to be transformed into CuAu—I face-centered square lattice to maximize the antiferromagnetic properties when the ratio of content between Pt and Mn is 50:50 in terms of atomic percent (at %). With this knowledge, the present inventors have made spin valve thin-film device having an antiferromagnetic layer composed of a Pt—Mn alloy, the content ratio between Pt and Mn being set substantially at 50:50, and measured the strength of the exchange magnetic field generated between the antiferromagnetic layer and a ferromagnetic layer. As a result, the inventors found that the strength of the exchange coupling magnetic field is still unsatisfactory, despite the use of the composition ratio between Pt and Mn which is ideal for a bulk. This is attributable to the fact that the transformation from a disordered lattice to an ordered lattice is still insufficient despite of the heat treatment.